Subsea communication

ABSTRACT

Methods of subsea communication in which a cathodic protection anode is used as a transmitting antenna. The signals are received at a second location by two electrodes which are spaced apart by different distances from the transmitting antenna.

BACKGROUND OF THE INVENTION

1. Field of the Inventon

This invention relates to subsea communication.

2. Description of Related Art

There are a wide variety of different circumstances in which it isdesirable to be able to communicate between bodies in a subseaenvironment. The term subsea is used in this application as this isconventional terminology, however, it should be appreciated that theterm subsea encompasses any underwater situation.

A specific example of where subsea communication is important is in theoperation of tetherless Autonomous Underwater Vehicles (AUVs) andtethered remotely operated vehicles (ROVs). Such vehicles need to becontrolled and the data which they gather needs to be extracted. It ispreferable if this control and extraction of data can be done on aregular basis (perhaps in real time) and without the need for a physicalconnection between the vehicle and, say, a surface vessel and withoutthe need to bring the vehicle back to the surface vessel.

Various means of communication between AUV's and other locations havebeen investigated. One possibility is acoustic transmission but thissuffers from difficulties because of noise generated by the fans whichdrive the AUV.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a means for subseacommunication which amongst other things, may be suitable for use withAUV's.

According to a first aspect of the present invention there is provided amethod of subsea communication comprising the steps of:

transmitting signals from a first location by causing subsea antennameans to emit electromagnetic radiation, and

receiving the signals at a second location via two electrodes disposedat different distances from the antenna means.

According to a second aspect of the present invention there is provideda subsea communication system comprising:

transmitting means disposed at a first location and arranged to causesubsea antenna means to emit electromagnetic signals,

two electrodes disposed at different distances from the antenna means,and receiving means disposed at a second location and arranged toreceive the signals via the electrodes.

According to a third aspect of the present invention there is providedsubsea communication apparatus for use in a subsea communication systemor method, the apparatus comprising transmitting means locatable at afirst location for causing subsea antenna means to emit electromagneticsignals, and receiving means locatable at a second location and arrangedto receive signals via two electrodes disposed at different distancesfrom the antenna means.

According to a fourth aspect of the present invention there is providedsubsea communication apparatus for use in a subsea communication systemor method, the apparatus comprising at least one subsea antenna means,transmitting means locatable at a first location for causing the subseaantenna means to emit electromagnetic signals, two electrodes locatableat different distances from the antenna means and receiving meanslocatable at a second location and arranged to receive signals via theelectrodes.

The communication method, system and apparatus can function because theelectric field seen at one of the electrodes is different from that seenat the other electrode due to the reduction in signal strength asdistance from the antenna is increased. Thus a signal is not measuredrelative to some earth but by virtue of a spatial change in fieldstrength. The maximum signal will be measured in a directionperpendicular to the field lines. Thus it is preferable if the twoelectrodes are arranged along a vector pointing towards the signalsource or the expected direction of the signal source.

The communication method, system and apparatus are particularlyapplicable in relation to pipeline systems, such as those used in theoil and gas industry. In such cases the first or second location mightbe part of a pipeline system and the respective other location might beon a surface vessel, a surface structure or a subsea vehicle. Similarlythe first or second location might be on a subsea vehicle and therespective other location might be on a surface vessel or a surfacestructure. Communication in accordance with this application can becarried out without the need for connecting cables. This, amongst otherthings, facilitates tetherless operation of subsea vehicles.

In a particular preferred implementation the first location is a remotewellhead. A remote wellhead is one which is not connected by a seabed(or other) flowline to a larger pipeline system. Such a well may be amonitoring well or a formerly producing well that has been cut off.

The antenna means may comprise a cathodic protection anode provided onmetallic structure.

This communication method, system and apparatus may be used to send datagathered from sensors or other components from the first to the secondlocation. In some embodiments the data might include measurements takenat a pipeline system and transmitted from the pipeline system. In such acase, the antenna means may comprise an anode. In other embodiments thedata might include information collected by an Autonomous UnderwaterVehicle (AUV), for example, the AUV might conduct a survey of a pipelinesystem and upload the survey data to a surface structure or vessel.

In yet further embodiments where signals are transmitted from a pipelinesystem, the pipeline system itself may be used as a signal channel tocarry data to be transmitted from a remote location to the region of theantenna means for emission. In one example, a AUV may be local to apipeline system and it may be desired to transmit data to the AUV from aplatform in the pipeline system. In such a case, the data can be sentalong the pipeline from the platform to the vicinity of the AUV and thentransmitted from antenna means to the AUV. This type of communicationmay be accomplished by receiving and retransmitting the data in theregion of the antenna or simply by causing or allowing the antenna toradiate the signal received along the pipeline.

At least one of the electrodes may comprise a portion of a structure,vessel or vehicle which contacts with a body of water through whichsignals are to be sent. Said portion may be a standard part of thestructure, vessel or vehicle or may be specially provided. The otherelectrode may be provided towards the distal end of an elongateconductor, for example a shielded cable.

Preferably the characteristics of the radiation, in particular thefrequency, and the difference in distance of the electrodes from theantenna means are chosen so that the drop in signal between theelectrodes caused by the fall off of the signal is significantly greaterthan differences in signal strength caused by phase difference. Inpractice this will be easily accomplished because the frequencies ofelectric signals which propagate well through water correspond towavelengths which are very large compared with any practical electrodespacing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

An embodiment of the present invention will now be described by way ofexample only, with reference to the accompanying drawing whichschematically shows a surface vessel, an AUV and part of a pipelinesystem in which the present invention is embodied.

DETAILED DESCRIPTION OF THE INVENTION

The drawing shows a pipeline system 1, provided at the sea bed, anAutonomous Underwater Vehicle (AUV) 2 located in the sea surrounding thepipeline system 1 and a surface vessel 3.

The pipeline system 1 comprises a production string 11 which terminatesat a wellhead 12. The wellhead 12 is provided with a cathodic protectionanode 13 which is in electrical contact with the metallic structure ofthe production string 11 via the wellhead 12. Sensors 14 for measuringparameters such as temperature and pressure are provided in theproduction string 11 and an output of the sensors 14 is connected to anelectronics module 15. In turn, the electronics module 15 is connectedto a toroidal coil 16 provided around the production string 11.

The electronics module 15 and toroidal coil 16 are arranged so that datareceived from the sensors 14 can be encoded onto signals which areinduced onto the production string 11 by a transformer like interaction.In essence, the metallic structure of the pipeline system 1 and an earthreturn form a single turn winding in a transformer onto which signalscan be induced. However, the mechanism for injection of signals onto theproduction string 11 is not of great significance to the presentapplication and so will not be described in any further detail.

The anode 13 is in electrical contact with the production string 11, viathe wellhead 12 and is uninsulated. Therefore, the anode 13 will act asan antenna means emitting radiation into the surrounding sea whichcorresponds to the signals injected onto the production string 11.

The AUV 2 comprises receiving means 21 which is connected to a pair ofspaced electrodes 22, 23. The first of these electrodes 21 is providedon the surface of the AUV 2 and the second electrode 23 is provided atthe end of a shielded cable 24.

The AUV 2 also comprises transmitting means 25 with an associatedtransmitting attena 26. Meanwhile the surface vessel 3 comprisesreceiving means 31 which is connected to a pair of spaced electrodes 32,33 which are similarly or identically arranged to those of the AUV 2.Thus, a first of the electrodes 32 is provided on the surface of thesurface vessel 3 and the second of the electrodes 33 is provided at theend of a shielded cable 34.

In general terms, each of the receiving means 21, 31 and its associatedelectrodes 22, 23; 32, 33 is arranged to receive signals emitted intothe water by a respective subsea antenna be this the anode 13 of thepipeline system 1 or the transmitting antenna of 26 of the AUV 2.

In both cases, the respective pairs of spaced electrodes 22, 23; 32, 33are at different distances from the respective transmitting antenna 13,26 and are able to pick up signals because the electric field seen atthe respective first electrode 22, 32 will be different from that seenat the respective second electrode 23, 33.

In this embodiment, data concerning the parameters measured by thesensors 14 may be transmitted by the anode 13 and received by the AUV 2and/or by the surface vessel 3.

Similarly, information gathered by the AUV 2, either via its respectivereceiving means 21 or by other equipment located on the AUV 2, may betransmitted to the surface vessel 3.

In one particular mode of operation, the AUV 2 is used to perform asurvey of a pipeline system, for example using sonar scanning equipment,and the results of this survey may be transmitted from the AUV 2 to thesurface vessel 3.

Although not shown in the drawings, the surface vessel 3 may alsocomprise suitable transmitting means for transmitting signals which canbe received at the AUV2 via its respective electrodes 22, 23 andreceiving means 21. This information might, for example, includeinstructions for controlling the path and/or operation of the AUV2.

In the present embodiment, electrical signals having frequencies in theorder of 100 Hz or a few hundred Hz are used and received via theelectrodes. In such circumstances it is expected that the system willhave a range of at least the order of low hundreds of meters but a rangein the order of a few Km is not out of the question. It should also benoted however, that different frequencies of radiation may be used.

The spacing between the first and second electrodes 32, 33 in the caseof a surface vessel 3 might be say 20 to 30 meters whereas when used onan AUV 2 the spacing might be 1 or 2 meters. In general a greaterspacing will improve the ability to pick up signals but practicalitieslimit the spacing.

In the embodiment described above, signals are transmitted from theanode 13 by way of direct transmission from the adjacent pipeline system1. However, in alternatives, a signal may be output via the anode 13 bytransmitting across inductance means provided in series between theanode 13 and the adjacent portion of the pipeline system 1.

Although the embodiment described above relates to the use of theinvention in a specific way in relation to pipeline systems, AUVs andsurface vessels, the communication method, system, and apparatus are notlimited to use in such a way.

For example, although the wellhead 12 described above is connected to apipeline P the present system still functions with, and in fact isparticularly attractive for use with remote wellheads which are notconnected to a larger pipeline system. Such wells may be monitoringwells or wells which are temporarily or permanently out of use and cutoff. Thus, in an alternative embodiment, the well head 12 is a remotewellhead, the remainder of the system being substantially the same asthat described above

In an extension of the present embodiment the first and secondelectrodes 22, 23, 32, 33 may be replaced with a 3 axis detection systemcomprising three orthogonally arranged pairs of spaced electrodes. Thiscan allow the direction from which the signal is being transmitted to bedetermined as this corresponds to the direction of maximum signalstrength. If this idea is further extended so that multiple signals fromdifferent origins are received and their directions determined, it ispossible using triagulation to determine the position of the receivingvessel, structure or vehicle relative to the transmitting origins.

1. A method of remote wellhead subsea communication comprising the stepsof: transmitting signals from a remote wellhead by causing a subseaantenna comprising a cathodic protection anode provided on the remotewellhead to emit electromagnetic signals; and receiving the signals at alocation away from the remote wellhead via two electrodes disposed atdifferent distances from the antenna.
 2. A method according to claim 1in which the location away from the wellhead is on one of a surfacevessel and a subsea vehicle.
 3. A method of subsea vehicle communicationcomprising the steps of: transmitting signals from a pipe line system bycausing a subsea antenna comprising a cathodic protection anode providedon the pipeline to emit electromagnetic signals; and receiving thesignals at a subsea vehicle via two electrodes disposed at differentdistances from the antenna.
 4. A method according to claim 3 includingthe further step of using the pipeline system as a signal channel tocarry data to be transmitted from a remote location to a region of theantenna for emission.
 5. A method according to claim 4 including thestep of receiving and re-transmitting the data in the region of theantenna.
 6. A method according to claim 4 including the step of allowingthe antenna to radiate the signal received along the pipeline.
 7. Amethod according to claim 1 comprising the further step of arranging thetwo electrodes along a vector pointing towards one of the signal sourceand an expected position of the signal source.
 8. A method according toclaim 3 in which one of the electrodes comprises a portion of one of avessel and a vehicle which contacts with a body of water through whichsignals are to be sent and the other electrode is provided towards thedistal end of an elongate conductor.
 9. A method according to claim 1 inwhich the signals are received via a 3 axis detection system comprisingthree orthogonally arranged pairs of spaced electrodes.
 10. A remotewellhead subsea communication system comprising: a transmitter disposedat a remote wellhead and arranged to cause a subsea antenna comprising acathode protection anode provided on the remote wellhead to emitelectromagnetic signals; two electrodes disposed at different distancesfrom the antenna; and a receiver disposed at a location away from thewellhead and arranged to receive the signals via the electrodes.
 11. Asubsea vehicle communication system comprising: a transmitter disposedon a pipeline and arranged to cause a subsea antenna comprising acathodic protection anode provided on the pipeline to emitelectromagnetic signals; two electrodes disposed at different distancesfrom the antenna; and a receiver disposed on a subsea vehicle andarranged to receive the signals via the electrodes.
 12. A method ofremote wellhead subsea communication comprising the steps of:transmitting signals from a remote wellhead by causing subsea antennameans comprising a cathodic protection anode provided on the remotewellhead to emit electromagnetic signals; and receiving the signals at alocation away from the remote wellhead via two electrodes disposed atdifferent distances from the antenna means.
 13. A method of subseavehicle communication comprising the steps of: transmitting signals froma pipe line system by causing subsea antenna means comprising a cathodicprotection anode provided on the pipeline to emit electromagneticsignals; and receiving the signals at a subsea vehicle via twoelectrodes disposed at different distances from the antenna means.
 14. Aremote wellhead subsea communication system comprising: transmittingmeans disposed at a remote wellhead and arranged to cause subsea antennameans comprising a cathode protection anode provided on the remotewellhead to emit electromagnetic signals; two electrodes disposed atdifferent distances from the antenna means; and receiving means disposedat a location away from the wellhead and arranged to receive the signalsvia the electrodes.
 15. A subsea vehicle communication systemcomprising: transmitting means disposed on a pipeline and arranged tocause subsea antenna means comprising a cathodic protection anodeprovided on the pipeline to emit electromagnetic signals; two electrodesdisposed at different distances from the antenna means; and receivingmeans disposed on a subsea vehicle and arranged to receive the signalsvia the electrodes.
 16. A method according to claim 3 comprising thefurther step of arranging the two electrodes along a vector pointingtowards one of the signal source and the expected position of the signalsource.
 17. A method according to claim 1 in which one of the electrodescomprises a portion of one of a vessel and a vehicle which contacts witha body of water through which signals are to be sent and the otherelectrode is provided towards the distal end of an elongate conductor.18. A method according to claim 3 in which the signals are received viaa 3 axis detection system comprising three orthogonally arranged pairsof spaced electrodes.
 19. A method of remote wellhead subseacommunication comprising the steps of: transmitting signals throughwater from a remote wellhead by causing a subsea antenna comprising acathodic protection anode provided on the remote wellhead to emitelectromagnetic signals; arranging two electrodes at selected positionssuch that the two electrodes are at different distances from theantenna; and receiving the signals at a location away from the remotewellhead via the two electrodes.
 20. A method according to claim 19comprising the step of selecting the positions of the two electrodes independence on an expected position of the signal source.
 21. A methodaccording to claim 7 comprising the step of varying the position of atleast one of the two electrodes in dependence on one of: the position ofthe signal source, and an expected position of the signal source.
 22. Amethod according to claim 16 comprising the step of varying the positionof at least one of the two electrodes in dependence on one of: theposition of the signal source, and an expected position of the signalsource.